Dimmer unit for a lighting control system

ABSTRACT

A plurality of dimmer units are connected between an alternating-current power source and different ones of a plurality of lamp loads for separately controlling the flow of alternating current through such lamp leads. Each dimmer unit includes a switching mechanism for passing the alternating current to the lamp load and a controllable direct-current responsive timing mechanism for controlling the fraction of each alternating-current half cycle during which the switching mechanism is conductive. The direct-current control signal input terminals of each dimmer unit are connected between corresponding output terminals of a pair of multiple output pre-set control units. A fader control unit supplies direct-current voltages to the pre-set control units and includes means for varying such voltages in an inverse manner for shifting control of the system from one pre-set control unit to the other. Each pre-set control unit includes a plurality of adjustable voltage dividers for individually determining the fractions of the pre-set unit input voltage appearing at the different ones of the pre-set unit output terminals.

United States Patent 1 Gilbreath [111 3,733,528 1 May 15,1973

I541 DIMMER UNIT FOR A LIGHTING CONTROL SYSTEM [75] Inventor: BenjaminF. Gilbreath, Richardson,

Tex.

[73] Assignee: Hunt Electronics Company, Dallas,

Tex.

[22] Filed: Aug. 2, 1971 [21] Appl. No.1 168,317

Primary Examiner-Alfred L. Brody Attorney-Giles C. Clegg, Jr. et al.

[57] ABSTRACT A plurality of dimmer units are connected between analternating-current power source and different ones of a plurality oflamp loads for separately controlling the flow of alternating currentthrough such lamp leads. Each dimmer unit includes a switching mechanismfor passing the alternating current to the lamp load and a controllabledirect-current responsive timing mechanism for controlling the fractionof each altermating-current half cycle during which the switchingmechanism is conductive. The direct-current control signal inputterminals of each dimmer unit are con-, nected between correspondingoutput terminals of a pair of multiple output pre-set control units. Afader control unit supplies direct-current voltages to the pre-setcontrol units and includes means for varying such voltages in an inversemanner for shifting control of the system from one pre-set control unitto the other. Each pre-set control unit includes a plurality ofadjustable voltage dividers for individually determining the fractionsof the pre-set unit input voltage appearing at the different ones of thepre-set unit output terminals.

7 Claims, 4 Drawing Figures DIMMER UNIT FOR A LIGHTING CONTROL SYSTEMBACKGROUND OF THE INVENTION This invention relates to lighting controlsystems and is particularly useful in connection with theatre lightingsystems, stage lighting systems and the like.

In theatre lighting systems, it is sometimes desired to vary theintensity of different banks of stage lights from one brightness levelto another and sometimes with the different banks being changed bydifferent amounts. In such cases, it is sometimes desired that two ormore of the lamp banks be changed simultaneously and that the change ineach case be a relatively smooth and more or less gradual one. As aconsequence, various forms of stage lighting control systems have beenheretofore proposed for accomplishing such purposes. In general,however, these previously proposed systems have suffered from one ormore limitations and disadvantages which it would be desirable tominimize or overcome. Some heretofore proposed systems are relativelycomplex and cumbersome, some are relatively expensive and inefficient,while others are difficult to operate and do not providemuch'flexibility in their manner of use.

It is an object of the invention, therefore, to provide a new andimproved lighting control system which is particularly useful fortheatrical lighting purposes and which avoids or minimizes one or moreof the limitations of systems heretofore proposed for this purpose.

It is another object of the invention to provide a new and improvedlighting control system for use in controlling multiple lamp loads in arelatively simple manner and with a relatively large degree offlexibility.

For a better understanding of the present invention, together with otherand further objects and features thereof, reference is had to thefollowing description taken in connection with the accompanyingdrawings, the scope of the invention being pointed out in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings:

FIG. 1 is a block diagram of a lighting control system constructed inaccordance with the present invention;

FIG. 2 is a detailed schematic circuit diagram of a fader control unitused in the FIG. 1 system;

FIG. 3 is a detailed schematic circuit diagram of one of the pre-setcontrol units used in the FIG. 1 system; and

FIG. 4 is a detailed schematic circuit diagram of one of the dimmerunits of the FIG. 1 system.

DESCRIPTION OF THE FIG. 1 SYSTEM Referring to FIG. 1, there is shown astage lighting control system having a plurality of controllable dimmerunits -13 connected between an alternatingcurrent power source (notshown) and different ones of a plurality of lamp loads 14-17. Each ofthe dimmer units 10-13 includes a pair of alternating-current inputterminals which are connected to a pair of altematingcurrent powersupply conductors 18 and 19 which, in use, are connected to analternating-current power line. The alternating-current input terminalsfor the dimmer unit 10 are indicated at 20 and 21. Each of the dimmerunits 10-13 further includes a pair of alternating-current outputterminals for connecting the dimmer unit to its lamp load. Thealternating-current output terminals for dimmer unit 10 are indicated at22 and 23. Each of the dimmer units 10-13 acts to control the fractionof each alternating-current half cycle during which current is allowedto flow from the altemating-current power supply conductors 18 and 19 toits lamp load. In this manner, by controlling the rootmean-square (r.m.s.) value of the current flow to the lamp load, the brightness orintensity of the lamp or lamps contained in the lamp load is controlled.

The lighting control system of FIG. 1 further includes a fader controlunit 24 having a set of three output terminals 25, 26 and 27 forsupplying separate but interdependent direct-current control voltages tothe input terminals of a pair of pre-set control units 28 and 29, theinput terminals for pre-set control unit 28 being indicated at 30 and 31and the input terminals for pre-set control unit 29 being indicated at32 and 33. For sake of explanation, fader unit output terminals 25, 26and 27 will be referred to as positive, common and negative outputterminals, respectively.

The first pre-set control unit 28 includes a plurality of outputterminals 34-37, while the second pre-set control unit 29 includes aplurality of output terminals 38-41.. Each of the dimmer units 10-13includes a pair of direct-current control voltage input terminals, oneof which is connected to one of the output terminals of the firstpre-set control unit 28 and the other of which is connected to one ofthe output terminals of the second pre-set control unit 29. Dimmer unit10, for example, includes a pair of direct-current control voltage inputterminals 42 and 43, the former being connected to output terminal 34 ofpre-set control unit 28 and the latter being connected to outputterminal 38 of pre-set control unit 29. Each of the pre-set controlunits 28 and 29 includes a plurality of adjustable voltage dividers forindividually determining the fractions of the pre-set unitdirect-current input voltage which is allowed to appear at the differentones of the pre-set unit output terminals. As such, the voltage dividerfor the first unit output terminal 34 may be set to determine the upperbrightness limit for the lamp load 14 controlled by the dimmer unit 10and the voltage divider for the second unit output terminal 38 set todetermine the lower or minimum brightness level for the lamp load 14 orvice versa.

F ader control unit 24 includes a direct-current power supply and anadjustable voltage divider for varying the magnitude of thedirect-current control voltage appearing between output terminals 25 and26 in an inverse manner with respect to the magnitude of thedirectcurrent control voltage appearing between out ut terminals 27 and26. In this manner, the voltage between terminals 27 and 26 is decreasedas the voltage between terminals 25 and 26 is increased or vice versa.With the fader unit voltage divider set at one end of its range, one ofthe voltages assumes a maximum value and the other assumes asubstantially zero value. When the fader unit voltage divider is set atthe other end of its range, the situation is reversed. Thus, byadjusting the fader unit voltage divider, control of the various lamploads 14-17 can be shifted from one of the pre-set control units 28 and29 to the other of such pre-set control units 28 and 29. This enablesthe simultaneous adjustment of the various lamp loads 14-17 such thatsome may be turned up brighter and others may be turned down dimmer orturned off in accordance with the settings of the voltage dividers inthe two pre-set control units 28 and 29.

In use, the fader control unit 24 and the pre-set control units 28 and29 may be mounted on a common control panel in a control booth, whilethe various dimmer units -13 may be located backstage close to the lamploads to be controlled. Both of the pre-set control units 28 and 29 areof an identical construction. All of the dimmer units 10-13 are also ofthe same construction. The number of dimmer units and lamp loads may beincreased or decreased as desired, four such dimmer units and lamp loadsbeing shown by way of example only.

DESCRIPTION OF THE FIG. 2 FADER CONTROL UNIT Referring now to FIG. 2,there is shown the details of the fader control unit 24 of the FIG. 1system. As seen in FIG. 2, the fader control unit 24 includes supplycircuit means for supplying a direct-current voltage. This supplycircuit means includes a regulated directcurrent power supply 50 whichis adapted to be coupled to and energized by an alternating-currentpower line by way of terminals 51 and 52. Power supply 50 produces aregulated direct-current voltage which appears between output terminals53 and 54 thereof, the former being of positive polarity and the latterof negative polarity. The fader unit 24 further includes adjustablevoltage divider means represented by a potentiometer 55 having endterminals 56 and 57 connected to the direct-current output terminals 53and 54, respectively, of the power supply 50. Potentiometer 55 furtherincludes an intermediate sliding contact 58.

The fader unit 24 also includes first and second direct-currentamplifier circuit means 60 and 61 for supplying selected fractions ofthe direct-current power supply voltage to the first fader unit outputterminal pair 25 and 26 and to the second fader unit output terminalpair 27 and 26. The first direct-current amplifier circuit 60 includes apair of n-p-n type transistors 62 and 63 coupled in cascade. The baseelectrode of the first transistor 62 is connected to the potentiometersliding contact 58, the emitter of the first transistor 62 is connectedto the base electrode of the second transistor 63 and the emitter of thesecond transistor 63 is connected to the common output terminal 26. Thecollector of the first transistor 62 is connected directly to the upperend terminal 56 of the potentiometer 55, while the collector of thesecond transistor 63 is connected to such upper end terminal 56 by wayof diode means represented by a pair of series connected diodes 64 and65. The collector of the second transistor 63 is also connected to thepositive fader unit output terminal 25.

The second direct-current amplifier circuit 61 includes a pair of p-n-ptype transistors 66 and 67 connected in cascade with one another. Thebase electrode of the first transistor 66 is connected to thepotentiometer sliding contact 58, the emitter of the first transistor 66is connected to the base electrode of the second transistor 67 and theemitter of the second transistor 67 is connected to the common outputterminal 26. The collector of the first transistor 66 is connecteddirectly to the lower end terminal 57 of the potentiometer 55, while thecollector of the second transistor 67 is connected to such lower endterminal 57 by way of diode means represented by series connected diodes68 and 69. The collector of the second transistor 67 is also connectedto the negative fader unit output terminal 27.

For an intermediate setting of the potentiometer sliding contact 58,direct current flows from the upper or positive power supply outputterminal 53, through diodes 64 and 65, transistors 63 and 67, diodes 68and 69 and back to the negative terminal 54 of the power supply 50.Except for the relatively small voltage drops across the diodes 64, 65,68 and 69, the entire output voltage of the power supply 50 appearsbetween the positive and negative fader unit output terminals 25 and 27.The fraction of this total voltage which appears between the positivevoltage output terminal pair 25 and 26, as compared to the fraction ofthe total voltage which appears between the negative voltage outputterminal pair 27 and 26, is determined by the setting of the slidingcontact 58. If sliding contact 58 is placed at the maximum upperposition so as to contact the upper end terminal 56, the uppertransistors 62 and 63 are turned full on and the lower transistors 66and 67 are turned off. This causes the positive output voltage betweenterminals 25 and 26 to assume a very nearly zero value and the negativeoutput voltage between terminals 27 and 26 to assume substantially themaximum or total value. If the sliding contact 58 is moved to itslowermost position so as to contact the lower end terminal 57, then thesituation is reversed. The positive output voltage between terminals 25and 26 assumes substantially the maximum value, while the negativeoutput voltage between terminals 27 and 26 assumes substantially a zerovalue. For intermediate settings of the sliding contact 58, the maximumavailable voltage is divided between the two outputs in accordance withthe setting of the sliding contact 58. As the positive output voltagebetween terminals 25 and 26 is increased, the negative output voltagebetween terminals 27 and 26 is decreased and vice versa so that thetotal of the two voltages always adds up to the maximum available value.Thus, adjustment of the sliding contact 58 serves to vary the two faderunit output voltages in an inverse manner.

Diodes 64 and 65 in the first amplifier circuit serve to produce avoltage drop to provide the necessary driving voltage for the baseelectrode of the second or output transistor 63 so that thecollector-toemitter voltage of such output transistor 63 may assume asubstantially zero value (approximately 0.1 volts) when the outputtransistor 63 is full on. If diodes 64 and were not used and thecollector of transistor 63 were connected directly to the power supplyterminal 53, the full on collector-to-emitter voltage 01 \utputtransistor 63 would be more on the order of a full volt, which value isobjectionable in the present system. Diodes 68 and 69 perform a similarfunction for the second amplifier circuit 61.

DESCRIPTION OF THE FIG. 3 PRE-SET CONTROL UNIT Referring now to FIG. 3,there is shown the details of the pre-set control unit 28 of the FIG. 1system. As seen in FIG. 3, the pre-set control unit 28 includes a firstvoltage supply line 70 which is connected to the positive voltagepre-set unit input terminal 30 by way of oppositely-poled diodes 71 and72, and a two-position switch 73. Pre-set unit 28 further includes asecond voltage supply line 74 which is connected directly to the commonpre-set unit input terminal 31. Switch 73 includes a movable Switchblade75 and fixed switch contacts 76 and 77. With switch 73 in theillustrated position (Switchblade 75 against contact 76), the presetcontrol unit 28 operates off of the positive output voltage of the fadercontrol unit 24, which voltage appears between pre-set input terminals30 and 31. When switch 73 is in the opposite position (Switchblade 75against contact 77), the pre-set control unit 28 operates off of aninternal direct-current voltage produced by a regulated direct-currentpower supply 78 which is adapted to be coupled to and energized by analternating-current power line. Among other things, the internal powersupply 78 enables the use of the pre-set control unit 28 without the useof the fader control unit 24.

The pre-set control unit 28 further includes a third voltage supply line80 which is connected to the positive voltage input terminal 30 by wayof a series connected p-n-p type transistor 81 and diode 82 and also byway of a series connected n-p-n type transistor 83 and diode 84. Thebase electrodes of both of the transistors 81 and 83 are connected tothe sliding contact 85 of a voltage dividing potentiometer 86, the endterminals of which are connected between pre-set unit voltage inputterminals 30 and 31.

The pre-set control unit 28 also includes a plurality of adjustablevoltage divider means represented by potentiometers 87-90 forindividually determining the fractions of the pre-set unit input voltageappearing at the different ones of the pre-set unit output terminals34-37. The end terminals of each of the potentiometers 87-90 areconnected between respective ones of threeposition switches 91-94 andthe lower or common voltage supply line 74. Potentiometers 87-90include, respectively, sliding contacts 95-98 which are individuallyconnected to different ones of the pre-set unit output terminals 34-37.Considering, for example, the switch 91 for the potentiometer 87, suchswitch 91 includes a movable Switchblade 99 and three stationarycontacts 100, 101 and 102 which are positioned for engagement thereby.Contact 100 represents a master control position and is connected to thesecond voltage supply line 80. Contact 101 represents an off positionand is not connected to anything. Contact 102 represents an individualcontrol position and is connected to the upper voltage supply line 70.The other switches 92-94 are constructed and connected in the samemanner as switch 91.

In use, the potentiometer switches 91-94 are set to the master position(e.g., contact 100 for switch 91) if it is desired that the lamp loadcontrolled by its potentiometer also be susceptible to control by acommon or master potentiometer 86. In other words, any of the lamp loadsconnected to any of the individual potentiometers 87-90 having theirswitches set to the master position can be simultaneously controlled bymovement of the sliding contact 85 of the master control potentiometer86. For the case of a positive voltage at input terminal 30, the settingof sliding contact 85 determines the conduction level or internalimpedance of the transistor 83 and hence the magnitude of thedirect-current voltage appearing between the second and common voltagesupply lines 80 and 74. Diode 84 serves as a blocking diode to preventreverse current flow through the transistor 83. If the voltage appearingat input terminal 30 should instead be of negative polarity, then thesetting of the master sliding contact 85 serves to determine theconduction level or internal impedance of the transistor 81 which, inthis case, determines the magnitude of the negative voltage appearingbetween the second and common voltage supply lines 80 and 74. Diode 82serves as a blocking diode to prevent reverse current flow through thetransistor 81. The oppositely-poled diodes 71 and72 in series with theupper voltage supply line function to match the voltage drops acrossblocking diodes 82 and 84 so that same net voltage will appear onvoltage supply line 70 as appears on the voltage supply line when thepotentiometer sliding contact is set to the upper end of thepotentiometer 86. The use of both positive-poled and negative-poledtransistors 81 and 83 and diodes 82 and 84 enables the same pre-setcontrol unit to be used for either positive or negative voltages on theinput terminal 30. For this reason, the second pre-set control unit 29can be and is of exactly the same construction as the first pre-setcontrol unit 28, even though the input voltage is of opposite polarity.

The lamp loads controlled by any of the potentiometers 87-90 havingtheir switches set to the off position (e.g., contact 101 for switch 91)will be turned off when the control potentiometer in fader unit 24 isset so that control of the lamp loads is determined only by the firstpre-set control unit 28 and not by the second pre-set control unit 29.The lamp loads controlled by any of the potentiometers 87-90 havingtheir switches set to the individual position (e.g., contact 102 forswitch 91) are removed from and not subjected to control by the masterpotentiometer 86. Such lamp loads are controlled only by the settings ofthe individual output potentiometers 87-90 and the setting of thecontrol potentiometer in the fader control unit 24.

By appropriate settings of the switches 91-94, some of the lamp loads14-17 can be subjected to simultaneous control by the masterpotentiometer 86, some can be turned off and some can be subjected tocontrol only by their individual ones of potentiometers 87-90 when thefirst pre-set control unit 28 is exclusively in control of the lamploads and the second pre-set control unit 29 is having no effectthereon. Similar considerations apply for the second pre-set controlunit 29. Thus, two different sets of stage lighting conditions can beestablished by the two pre-set control units 28 and 29 and the stagelighting shifted from one to the other by appropriate manipulation ofthe fader control potentiometer 55. Also, where multiple successivescenes require different lighting conditions, the light condition forthe next scene can be set up on the inactive pre-set control unit whilethe active unit creates the ghting condition for the existing scene.

DESCRIPTION OF THE FIG. 4 DIMMER UNIT Referring now to FIG. 4, there isshown the details of the dimmer unit 10 of FIG. 1. As seen in FIG. 4,the dimmer unit 10 includes circuit means for supplying analternating-current voltage. This circuit means is represented by inputterminals 20 and 21 which, in use, are connected to analternating-current power line or other source of alternating-currentpower.

The dimmer unit 10 also includes switching circuit means adapted to becoupled in series between the alternating-current supply terminals 20and 21 and the lamp load 14 for enabling current flow through the lampload 14 when such switching circuit means is conductive. This switchingcircuit means includes a pair of oppositely-poled unilateralsemiconductor switching devices 103 and 104 of the silicon-controlledrectifier type. Silicon-controlled rectifiers 103 and 104 are connectedby their anodes and cathodes in parallel with one another and one end ofthis parallel combination is connected to the alternating-current inputterminal 20 by way of a fuse 105. The other end of this parallelcombination is connected to the lamp load output terminal 23 by way ofthe low impedance primary winding 106 of a current transformer 107 and aradio-frequency choke coil 108. The other lamp load output terminal 22is connected directly to the second alternating-current input terminal21 by means of a conductor 109. A radio-frequency by-pass capacitor 110is connected between the first alternating-current input terminal 20 andthe lamp load output terminal 23. Choke coil 108 and capacitor 110constitute a radio-frequency filter for minimizing leakage to thealternating-current power line of transient-type radio-frequencycomponents generated by the switching action of siliconcontrolledrectifiers 103 and 104. The gate electrodes and cathodes of thesilicon-controlled rectifiers 103 and 104 are connected to differentones of a pair of secondary windings 111 and 112 of a pulse transformer113. The primary winding 114 for the pulse transformer 113 is shown nearthe lower right-hand comer of the drawing and, though shown in aseparated manner, is actually wound on the same core structure as arethe secondary windings 111 and 112. Siliconcontrolled rectifiers 103 and104 serve to control the fraction of each half cycle of thealternating-current line voltage during which current is allowed to flowfrom the alternating-current power line terminals 20 and 21 to the lampload 14.

The dimmer unit further includes a controllable timing mechanism forcontrolling the conduction intervals of the silicon-controlledrectifiers 103 and 104. This timing mechanism includes trigger circuitmeans for initiating conduction in the silicon-controlled rectifiers 103and 104 and timing circuit means for triggering such trigger circuitmeans during each half cycle of the alternating-current voltage. Suchtiming circuit means is responsive to the direct-current control voltagesupplied to dimmer unit direct-current control terminals 42 and 43 forcontrolling the trigger timing in accordance with the magnitude of suchdirect-current control voltage. The timing circuit means includes atiming capacitor 115 and the trigger circuit means includes asemiconductor switching device or trigger device represented by aprogrammable unijunction transistor 116, the latter being coupled acrossthe timing capacitor 115 for discharging such timing capacitor 115 whenthe voltage thereacross exceeds a predetermined threshold levelestablished by the programmable unijunction transistor 116. The triggercircuit means further includes circuit means represented by the pulsetransformer 113 for sensing the flow of capacitor discharge currentthrough the programmable unijunction transistor 1 16 and rendering theappropriate one of the silicon-controlled rectifiers 103 and 104conductive at the onset of such capacitor discharge current. One end ofthe pulse transformer primary winding 114 is connected by way of a diode117 to the cathode of the programmable unijunction transistor 116, whilethe other end of the pulse transformer primary winding 114 is connectedto a common current return line 118 for the control portion of thedimmer unit 10. A further diode 119 is connected across the primarywinding 114 for damping any reverse polarity spikes that might beproduced by the primary winding 114.

The timing circuit means also includes rectifier circuit means forrectifying the alternating-current voltage appearing betweenalternating-current input terminals 20 and 21. This rectifier circuitmeans includes an isolation transformer 120 and a diode bridge typefullwave rectifier circuit 121. Output terminals 122 and 123 ofrectifier circuit 121 are connected to a voltage supply line 124 and thecommon current return line 118, respectively, the voltage at terminal122 being of positive polarity relative to the voltage at terminal 123.There thus appears on voltage supply line 124 a continuous succession ofpositive-going sinusoidal half cycles representing a full-wave rectifiedversion of the alternating-current voltage appearing between dimmer unitalternating-current input terminals 20 and 21.

The timing circuit means further includes first charging circuit meansresponsive to the rectified alternating-current voltage appearing on thevoltage supply line 124 for charging the timing capacitor 115 at a firstrate. This first charging circuit means includes an adjustable resistor125 and a fixed resistor 126 connected in series between the voltagesupply line 124 and the upper end of the timing capacitor 115.

The dimmer unit 10 further includes circuit means for supplying theretoa direct-current control voltage and second charging circuit meansresponsive to such direct-current control voltage for charging thetiming capacitor 115 at a second rate. The circuit means for supplying adirect-current control voltage includes the direct-current inputterminals 42 and 43 which, as seen in FIG. 1 are connected to thepre-set control units 28 and 29 for receiving the direct-currentvoltages appearing at the output terminals 34 and 38 of such pre-setcontrol units 28 and 29. The second charging circuit means includes ann-p-n type transistor 130 having its collector connected to a regulatedvoltage supply line 131 by way of a resistor 132 and having its emitterconnected to the common current return line 118 by way of a resistor133. The emitter of transistor 130 is also coupled to the upper end ofthe timing capacitor 115 by means of a diode 134. The direct-currentcontrol voltage input terminals 42 and 43 are connected to the baseelectrode of the transistor 130 by a voltage divider circuit formed byresistors 135 and 136 and diodes 137 and 138. A further resistor 139 isconnected between the negative input terminal 43 and the regulatedvoltage supply line 131, while a thermistor 140 is connected between thenegative terminal 43 and th common current return line 1 18. Aradio-frequency r v-pass capacitor 141 is connected between the twoinput terminals 42 and 43 for filtering out any undesired radiofrequencynoise which may be supplied to the input terminals 42 and 43.

The dimmer unit 10 further includes a regulated direct-current powersupply circuit for supplying regulated direct-current voltage to theregulated voltage supply line 131. This power supply circuit includes aresistor 142, a diode 143 and a filter capacitor or smoothing capacitor144 which are connected in series between the output terminals 122 and123 of the bridge rectifier circuit 121. This power supply circuitfurther includes a Zener diode 145 connected across the diode 143 andsmoothing capacitor 144. Zener diode 145 provides the regulating actionand, in effect, clamps the peak voltage at point 146 at a predeterminedlevel of, for example, 25 volts. Capacitor 144 serves to smooth out theundulations in the rectified and peak limited voltage to provide on thevoltage supply line 131 a constant amplitude regulated direct-currentvoltage.

The dimmer unit further includes circuit means responsive to thealternating-current power line voltage for developing a direct-currentbias voltage proportional to the peak amplitude thereof and forsupplying same to the gate electrode of the trigger device representedby the programmable unijunction transistor 116 for controlling thethreshold level of such trigger device 116. This threshold controlcircuit means includes a resistor 147, a diode 148 and a smoothingcapacitor 149 connected in series between the rectified voltage supplyline 124 and the common current return line 118. Diode 148 and capacitor149 function as a peak detector to develop across the capacitor 149 adirect-current voltage proportional to the peak value of the rectifiedvoltage on supply line 124 and, hence, proportional to the peak value ofthe alternating power line voltage appearing between pre-set unit inputterminals and 21. The threshold control circuit further includes apotentiometer 150 and a fixed resistor 151 connected in series acrossthe smoothing capacitor 149. A sliding tap 152 on potentiometer 150 isconnected to the gate electrode of the programmable unijunctiontransistor 116.

The dimmer unit 10 further includes additional switching circuit meanscoupled across the timing capacitor 115 and responsive to the rectifiedalternatingcurrent voltage on voltage supply line 124 for completelydischarging the timing capacitor 115 at the end of each half cycle ofthe alternating-current power line voltage. For sake of explanation,such additional switching circuit will be referred to as a line synccircuit. Such line sync circuit functions to insure that the dischargingof the timing capacitor 115 is sychronized with the timing of thealternating-current power line voltage at terminals 20 and 21 so thatthe timing capacitor 1 15 will be completely discharged at the beginningof each half cycle of such alternating-current power line voltage. Thisline sync circuit includes a pair of cascade coupled n-p-n typetransistors 154 and 155, the collector of the former being connected tothe base electrode of the latter. Transistor 155 is connected across thetiming capacitor 115 by means of its collector and emitter electrodes.The collector of the first transistor 154 is connected to the regulatedvoltage supply line 131 by way of a resistor 156 and the emitter of suchtransistor 154 is connected directly to the common current return line118. The base electrode of the first transistor 154 is connected to apoint intermediate a pair of voltage dividing resistors 157 and 158which are connected in series between the power supply junction point146 and the common current return line 118. The negative-going voltagespike appearing at this junction point 146 at the end of eachalternating current half cycle serves to momentarily turn off thenormally conductive first transistor 154 which, in turn, momentarilyturns on the normally non-conductive second transistor 155.

The dimmer unit 10 further includes an overload protection system forpreventing damage to the siliconcontrolled rectifiers 103 and 104 andother components in the lamp load circuit in the event that excessivecurrent should be drawn by the lamp load 14. As will be seen, thisoverload protection system includes means for protecting against bothexcessive values of average current flow as well as excessive values ofpeak current flow to the lamp load 14. This overload protection systemincludes circuit means for sensing the alter nating current flowing tothe lamp load 14 and developing a voltage signal proportional to theinstantaneous amplitude thereof. This current sensing circuit meansincludes. the current transformer 107 and a diode bridge type full-waverectifier circuit 160. The low impedance primary winding 106 of thecurrent transformer 107 is connected in series between thesiliconcontrolled rectifiers 103 and 104 and the lamp load 14, while thesecondary winding 161 of the current transformer 107 is connected acrossa first diagonal of the bridge circuit 160. An adjustable load resistor162 is connected between output terminals 163 and 164 of the bridgecircuit 160. Terminal 163 is also connected to the common current returnline 118. The polarity of the diodes in bridge circuit are such that thevoltage fluctuations appearing at output terminal 164 are all ofpositive polarity, such fluctuations corresponding in waveform to afull-wave rectified version of the current fluctuations passing throughthe primary winding 106 of the current transformer 107.

The overload protection system further includes circuit means coupled tothe current sensing circuit means represented by transformer 107 andbridge circuit 160 for clamping the dimmer unit 10 at a predeterminedoperating level when the average value of the current flowing to lampload 14 exceeds a predetermined value. This average overload circuitmeans includes a resistor 165, a diode 166 and a filter capacitor 167connected in series between bridge circuit output terminal 164 and thecommon current return line 118. Diode 166 and capacitor 167 function asa detector circuit to develop across the capacitor 167 a direct-currentvoltage proportional to the average value of the alternating currentflowing to the lamp load 14. By average value is meant the average valueover a half cycle, as opposed to a full cycle, or, more accurately, theaverage value over several cycles with the assumption that all halfcycles are of the same polarity. The average overload circuit furtherincludes a voltage dividing potentiometer 168 having a sliding contact169 connected to the base electrode of a p-n-p type transistor 170. Theemitter of transistor 170 is connected to the regulated voltage supplyline 131 by way of a resistor 171, while the collector of transistor 170is connected to the current return line 118 by way of a resistor 172.The collector of transistor 170 is further connected to the baseelectrode of the charging circuit transis or 130 by way of a diode 173.As will be seen, the a erage overload circuit serves to clamp thevoltage level at junction point 174 at a fixed predetermined valuewhenever the average value of the lamp load current exceeds a desiredmaximum level.

The overload protection system further includes circuit means coupled tothe current sensing circuit means represented by transformer 107 andrectifier 160 for disabling the dimmer unit 10 when the peak value ofthe current flowing to the lamp load 14 exceeds a predetermined value.This peak overload circuit includes a semiconductor breakover device inthe form of a silicon symmetrical switch 175 which is coupled to therectifier output terminal 164 by way of a diode 176. The other end ofthe silicon symmetrical switch 175 is connected to the common currentreturn line 118. The

silicon symmetrical switch 175 is a bilaterial diode switch device whichhas a fairly high internal impedance until the voltage thereacrossexceeds a breakover level, following which the internal impedance of thedevice 175 becomes relatively small. Thus, when one of theunidirectional pulses at the rectifier output terminal 164 exceeds thebreakover level of the device 175, such device becomes conductive andeffectively shorts the junction point 177 to the current return line118. This junction point 177 is connected to the timing capacitor 115 byway of conductor wire 178, diode 179 and conductor wire 180. Thus, whenconductive, the device 175, in effect, shorts out the timing capacitor115. This disables dimmer unit and prevents further current flow to thelamp load 14. Holding current for maintaining the device 175 conductiveonce the breakover level has been exceeded is provided by way ofconductor wire 178, resistor 181 and diode 182, the latter beingconnected to the regulated voltage supply line 131.

When the silicon symmetrical switch 175 is conductive, a warning lamp183 is turned on. Current for energizing the warning lamp 183 flows fromthe rectified voltage supply line 124 by way of a resistor 184,conductor wire 185, the lamp 183, conductor wire 186 and the siliconsymmetrical switch 175 to the common current return line 118. The peakoverload circuit may be reset by momentarily depressing a spring-loadedpushbutton switch 187 for purposes of momentarily closing same. Thisshorts out the silicon symmetrical switch 175 and turns same off.

OPERATION OF THE FIG. 4 DIMMER UNIT Considering now the operation of thedimmer unit 10, the power switching mechanism represented bysilicon-controlled rectifiers 103 and 104 serves to control the fractionof each half cycle during which current is allowed to flow from thealternating-current power line terminals 20 and 21 to the lamp load 14,siliconcontrolled rectifier 103 allowing current flow in one directionand silicon-controlled rectifier 104 allowing current flow in theopposite direction. Siliconcontrolled rectifiers 103 and 104 aretriggered by pulses produced by the programmable unijunction transistor116 and supplied to the gate electrodes of such silicon-controlledrectifiers 103 and 104 by the pulse transformer 113. Each pulse triggersthe siliconcontrolled rectifier which at that moment has a positivevoltage on its anode and a negative voltage on its cathode, suchsilicon-controlled rectifier thereafter being turned off when thealternating-current voltage reverses polarity.

The programmable unijunction transistor 116 is normally non-conductive.It becomes conductive and generates a current flow pulse when its anodevoltage exceeds its gate electrode voltage. Thus, the setting of slidingcontact 152 on the peak detector circuit potentiometer 150 establishesthe voltage threshold level at which the timing capacitor 115 will bedischarged by the programmable unijunction transistor 116.

At the beginning of each half cycle of the alternatingcurrent power linevoltage at terminals 20 and 21, the timing capacitor 115 is completelydischarged. If this was not accomplished by the programmable unijunctiontransistor 116, it will have been accomplished by the line sync circuitprovided by transistors 154 and 155. Transistor 154 is normallyconductive and transistor 155 is normally non-conductive. At the end ofeach half cycle of the power line voltage, there appears at the powersupply junction point 146 a negative-going voltage spike produced whenthe pulsating voltage on the voltage supply line 124 falls below thebreakover level of the Zener diode 145. This negative-going voltagespike is supplied by way of resistor 157 and momentarily turns off thefirst line sync transistor 154. This momentarily turns on the secondtransistor 155 and the resulting collector-to-emitter current flowtherethrough discharges the timing capacitor 115.

During the first portion of each half cycle, the timing capacitor ischarged by two different charging circuit mechanisms, one of which isresponsive to the direct-current control voltage applied across thedimmer unit input terminals 42 and 43 and the other of which isresponsive to the pulsating full-wave rectified voltage appearing onvoltage supply line 124. For convenience, the control voltage responsivemechanism will be referred to as the fast charge mechanism and therectified voltage responsive mechanism will be referred to as the slowcharge mechanism.

Considering first the fast charge mechanism, the timing capacitor 115 ischarged at a relatively rapid rate by the flow of direct current fromthe regulated voltage supply line 131, through resistor 132, through thecollector-to-emitter portion of transistor and through diode 134 tocapacitor 115. The actual charge rate is determined by the internalimpedance of the transistor 130 which is, in turn, determined by themagnitude of the direct-current control voltage applied across dimmerunit input terminals 42 and 43. These input terminals 42 and 43 areconnected to the base electrode of transistor 130 by the voltage dividerformed by resistors 135 and 136. The greater the magnitude of thedirect-current control voltage, the lower the internal impedance of thetransistor 130 and the faster the rate of charging of the timingcapacitor 1 15. As will be seen, the bias on the gate electrode of theprogrammable unijunction transistor 116 is such that the charging of thetiming capacitor 115 by the fast charge transistor 130 alone will not besufficient to trigger the programmable unijunction transistor 116.

To the fast charge voltage being built up across the timing capacitor 115, there is added a slow charge voltage supplied thereto by the slowcharge mechanism which is represented by resistors 125 and 126. Each ofthe positive-going half cycles of the pulsating voltage on supply line124 causes an approximately square law charging of the timing capacitor115. For simplicity of understanding, this sine wave charging can be*iought of as taking over and continuing the charging of the timingcapacitor 115 following completion of the initial fast charge applied byway of fast charge transistor 130. This additional slow charge componentcontinues until the voltage across timing capacitor 115 exceeds thethreshold level of the programmable unijunction transistor 116. At thispoint, the programmable unijunction transistor 116 becomes conductiveand the timing capacitor 115 is very quickly discharged by way of suchtransistor 116 and the primary winding 114 of pulse transformer 113. Thepulse produced by this sudden flow of capacitor discharge currentthrough the primary winding 114 triggers the appropriate one of thesilicon-controlled rectifiers 103 and 104.

The larger the magnitude of the direct-current control voltage atterminals 42 and 43, the quicker is the timing capacitor 115 charged upto the threshold level and the sooner in each half cycle is fired theappropriate one of silicon-controlled rectifiers 103 and 104. Thus, thegreater is the fraction of each half cycle during which current flowsthrough the lamp load 14 and, hence, the greater is the brightness ofthe lamps in lamp load 14.

In order to provide a preliminary setup adjustment for the dimmer unit10, the maximum value of directcurrent control voltage is appliedbetween the input terminals 42 and 43. The sliding contact 152 on thepotentiometer 150 is then adjusted to adjust the bias on the gateelectrode of the programmable unijunction transistor 116 to give themaximum desired brightness level (usually full on") for the lamp load14. Next, the voltage difference between input terminals 42 and 43 isset to zero and the adjustable resistor 125 in the slow charge circuitis adjusted so that the programmable unijunction transistor 116 justbarely does not fire during each half cycle. In other words, resistor125 is adjusted so that the timing capacitor 115 will be charged up bythe sine wave charging to just slightly less than the threshold level ofunijunction transistor 116 during the course of a complete half cycle.Lack of triggering of the programmable unijunction transistor 116 keepscurrent from flowing to the lamp load 14 and, hence, causes the lampload 14 to be turned off.

Variations in the peak amplitude of the alternating power line voltageat dimmer unit input terminals and 21 will cause correspondingvariations in the peak amplitude of the pulsating rectified voltage onvoltage supply line 124. This, in turn, will cause undesired variationsin the time required for the slow charge circuit to charge the timingcapacitor 115 up to the unijunction transistor threshold level. Such anundesired change in the trigger timing is offset and compensated for byan automatic and corresponding change in the magnitude of the biasvoltage supplied to the gate electrode of the unijunction transistor116. Thus, if the alternating-current line voltage amplitude increases,which increase would cause an earlier firing of the unijunctiontransistor 116, the bias voltage on the gate electrode of unijunctiontransistor 116 is increased a proportionate amount, which increase wouldcause a later firing of the unijunction transistor 116. These twofactors offset one another so that the unijunction transistor 116 isfired at the desired time.

Diodes 137 and 138 in the direct-current control voltage input circuitof the fast charge transistor 130 serve to provide a further increase inthe charge rate for very low values of direct-current control voltage.In other words, until the internal threshold or breakover level of thediodes 137 and 138 is exceeded, such diodes remain essentiallynon-conductive and the full value of the direct-current control voltageis applied to the base electrode of the fast charge transistor 130. Whensuch internal thresholds are exceeded and the diodes 137 and 138 becomeconductive, the voltage divider formed by resistors 135 and 136 providea voltage dividing action such that the control voltage applied to thebase electrode of transistor 130 corresponds to approximately one-halfof the control voltage value appearing between input terminals 42 and43. Thus, for very low values of direct-current control voltage, an evenfaster charge rate is employed by the fast charge circuit. This makesthe control voltage amplitude versus lamp load brightness curve morenearly square law in nature.

Thermistor 140 provides temperature compensation to compensate forchanges with temperature in the operating characteristics of the fastcharge transistor 130, the diode 134 and the programmable unijunctiontransistor 116.

Considering now the overload protection portion of the dimmer unit 10,the current transformer 107 senses the current flow to the lamp load 14and produces across the secondary winding 161 voltage pulsesproportional in amplitude to the amplitude of the lamp load currentpulses. These voltage pulses are full-wave rectified by the bridgerectifier circuit so that all of the pulses appearing at the outputterminal 164 thereof are of positive polarity.

The average overload portion of the overload protection system sensesthe average value of the positive polarity pulses at rectifier outputterminal 164 and clamps the base electrode of the fast charge transistor130 at a predetermined and relatively small voltage level when suchaverage value exceeds a desired maximum value. More particularly, filtercapacitor 167 receives the positive polarity pulses appearing atrectifier output terminal 164 and integrates or averages same to produceacross such capacitor 167 a voltage proportional to the average valuethereof and, hence, to the average value of the lamp load current. Whenthe lamp load current is below the overload level, the voltage developedacross filter capacitor 167 is not sufficient to turn off the normallyconductive transistor 170. With the transistor 170 conductive, there isdeveloped across the resistor 172 a voltage which keeps the clampingdiode 173 turned off. When the lamp load current exceeds the overloadlevel, on the other hand, the resulting average voltage developed acrossthe filter capacitor 167 turns off the transistor 170. This decreasesthe voltage drop across collector resistor 172 and turns on the clampingdiode 173. This enables such diode 173 to clamp the base electrode ofthe fast charge transistor 130 to reduce the fast charge to a maximumsafe value and to prevent such safe value from being exceeded by eitherthe existing or subsequent values of the directcurrent control voltageapplied to the input terminals 42 and 43. The threshold level for thisaverage overload circuit is set at the desired value by properadjustment of the sliding contact 169 of the potentiometer 168.

Considering now the peak overload portion of the overload protectionsystem, the silicon symmetrical switch 175 serves to monitor the peakvalue of t? a positive pulses appearing at rectifier output termin-l164. The breakover level of silicon symmetrical switch 175 is equal tothe peak voltage value at the rectifier output terminal 164corresponding to the maximum peak value of current flow which is desiredthrough the lamp load 14. When the peak amplitude of one or more of thepulses at rectifier output terminal 164 exceed this breakover level, thesilicon symmetrical switch 175 becomes conductive and provides a verylow impedance connection between the junction point 177 and the commoncurrent return line 118. Since the silicon symmetrical switch 175 isconnected in parallel with the timing capacitor 115 by means ofconductor 178, diode 179 and conductor 180, this effectively shorts outthe timing capacitor 115 and prevents any further charging thereof. Thisprevents further triggering of the siliconcontrolled rectifiers 103 and104 and, hence, prevents further current flow to the lamp load 14.Holding current for keeping the silicon symmetrical switch 175conductive is supplied thereto from the regulated power supply 142-145by way of diode 182, resistor 181 and conductor 178.

When the silicon symmetrical switch 175 is conductive, the peak overloadwarning lamp 183 is lit by current flow by way of resistor 184,conductor 185, lamp 183, conductor 186 and silicon symmetrical switch175. The peak overload circuit may be reset by momentarily depressing orclosing the spring-loaded pushbutton switch 187. This shorts out thesilicon symmetrical switch 175 and turns same off. Diodes 176 and 182prevent any reverse current flow therepast by way of the warning lamp183 when the silicon symmetrical switch is not conductive.

While there has been described what is at present considered to be apreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,intended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. A dimmer unit for use in a lighting control system and comprising:

circuit means for supplying an alternating-current voltage;

circuit means for supplying a direct-current control voltage;

switching circuit means adapted to be coupled in series between thealternating-current supply circuit means and a lamp load for enablingcurrent flow to the lamp load when such switching circuit means is Iconductive;

a timing capacitor;

a trigger device coupled to the timing capacitor for discharging sameand supplying a trigger pulse to the switching circuit means when thecharge on the timing capacitor exceeds a predetermined threshold level;and controllable charging circuit means responsive to the direct-currentcontrol voltage for charging the timing capacitor at a rate dependent onthe magnitude of such direct-current control voltage.

2. A dimmer unit in accordance with claim 1 wherein the trigger deviceis a programmable unijunction transistor.

3. A dimmer unit in accordance with claim 1 and including:

means for controlling the threshold level of the trigger device;

and circuit means responsive to the alternating- LII current voltage fordeveloping a direct-current bias voltage proportional to the peakamplitude thereof and for supplying such bias voltage to the triggerdevice threshold control means for compensating for changes in magnitudeof the alternating-current voltage.

4. A dimmer unit in accordance with claim 3 wherein the trigger deviceis a programmable unijunction transistor having anode, cathode and gateelectrodes, the anode and cathode electrodes being coupled across thetiming capacitor and the gate electrode being coupled to the circuitmeans for developing the direct-current bias voltage.

5. A dimmer unit for use in a lighting control system and comprising:

circuit means for supplying an alternating-current voltage;

circuit means for supplying a direct-current control voltage;

first switching circuit means adapted to be coupled in series betweenthe alternating-current supply circuit means and a lamp load forenabling current flow to the lamp load when such switching circuit meansis conductive;

capacitor means;

second switching circuit means coupled across the capacitor means fordischarging same when the voltage thereacross exceeds a predeterminedthreshold level; circuit means for sensing the flow of capacitordischarge current and rendering the first switching circuit meansconductive at the onset thereof;

rectifier circuit means for rectifying the'alternatingcurrent voltage;

first charging circuit means responsive to the rectifiedalternating-current voltage for charging the capacitor means at a firstrate;

and second charging circuit means responsive to the direct-currentcontrol voltage for charging the capacitor means at a second rate.

6. A dimmer unit in accordance with claim 5 and including thirdswitching circuit means coupled across the capacitor means andresponsive to the rectified alternating-current voltage for completelydischarging the capacitor means at the end of each half cycle of thealternating-current voltage.

7. A dimmer unit in accordance with claim 6 and including circuit meansresponsive to the rectified alternating-current voltage for developing adirect-current bias voltage proportional to the peak amplitude thereofand for supplying such bias voltage to the second switching circuitmeans for controlling the th eshold level thereof for compensating forchanges in magnitude of the alternating-current voltage.

1. A dimmer unit for use in a lighting control system and comprising:circuit means for supplying an alternating-current voltage; circuitmeans for supplying a direct-current control voltage; switching circuitmeans adapted to be coupled in series between the alternating-currentsupply circuit means and a lamp load for enabling current flow to thelamp load when such switching circuit means is conductive; a timingcapacitor; a trigger device coupled to the timing capacitor fordischarging same and supplying a trigger pulse to the switching circuitmeans when the charge on the timing capacitor exceeds a predeterminedthreshold level; and controllable charging circuit means responsive tothe direct-current control voltage for charging the timing capacitor ata rate dependent on the magnitude of such direct-current controlvoltage.
 2. A dimmer unit in accordance with claim 1 wherein the triggerdevice is a programmable unijunction transistor.
 3. A dimmer unit inaccordance with claim 1 and including: means for controlling thethreshold level of the trigger device; and circuit means responsive tothe alternating-current voltage for developing a direct-current biasvoltage proportional to the peak amplitude thereof and for supplyingsuch bias voltage to the trigger device threshold control means forcompensating for changes in magnitude of the alternating-currentvoltage.
 4. A dimmer unit in accordance with claim 3 wherein the triggerdevice is a programmable unijunction transistor having anode, cathodeand gate electrodes, the anode and cathode electrodes being coupledacross the timing capacitor and the gate electrode being coupled to thecircuit means for developing the direct-current bias voltage.
 5. Adimmer unit for use in a lighting control system and comprising: circuitmeans for supplying an alternating-current voltage; circuit means forsupplying a direct-current control voltage; first switching circuitmeans adapted to be coupled in series between the alternating-currentsupply circuit means and a lamp load for enabling current flow to thelamp load when such switching circuit means is conductive; capacitormeans; second switching circuit means coupled across the capacitor meansfor discharging same when the voltage thereacross exceeds apredetermined threshold level; circuit means for sensing the flow ofcapacitor discharge current and rendering the first switching circuitmeans conductive at the onset thereof; rectifier circuit means forrectifying the alternating-current voltage; first charging circuit meansresponsive to the rectified alternating-current voltage for charging thecapacitor means at a first rate; and second charging circuit meansresponsive to the direct-current control voltage for charging thecapacitor means at a second rate.
 6. A dimmer unit in accordance withclaim 5 and including third switching circuit means coupled across thecapacitor means and responsive to the rectified alternating-currentvoltage for completely discharging the capacitor means at the end ofeach half cycle of the alternating-current voltage.
 7. A dimmer unit inaccordance with claim 6 and including circuit means responsive to therectified alternating-current voltage for developing a direct-currentbias voltage proportional to the peak amplitude thereof and forsupplying such bias voltage to the second switching circuit means forcontrolling the threshold level thereof for compensating for changes inmagnitude of the alternating-current voltage.